I have 18 year old, Anderson double hung (builder grade) windows in my house. The other day, as I was closing one of them it sounded like a spring broke inside the window/frame. Now the window is extremely difficult to close.

any idea what broke and can it be repaired? how difficult of a job will be be?

There is a triangle etched in the corner of the glass with the Andersen logo and some numbers/letters. Give that and the glass size to Andersen, they can supply the correct balance. It is usually the cable that breaks(nylon), and it should come with instructions.